ch 10-11

• Information contained in genes that gets passed onto new generation
• Characteristics of Genetic Material

  • Expression of information (phenotype)
  • Storage of complex information
  • Capacity for variation
  • Replication
• Throughout the early to mid-1900s, debate over the identity of genetic material focused on the DNA and proteins
• At the start, protein was favored -> proteins are very common in cells and with 20 amino acids, proteins were thought to have a much higher capacity for variation than the 4 nucleotide DNA
• 3 crucial experiments showed otherwise
• Griffith Transformation Experiments


  Showed avirulent strains of Streptococcus pneumoniae could be transformed to virulence
• Avery, MacLeod, and McCarty Transformation Experiment

  Demonstrated transforming principle was DNA, not protein
• Hersey-Chase Phage Experiments


  Showed DNA, not protein, is the genetic material responsible for heredity in bacteriophages
• Griffith's transformation experiment showed that a chemical component of cells could introduce a new trait to a cell
• Avery et al. (1944) result was met with a mixture of excitement and skepticism
• Hershey was unconvinced by the results of his own experiment
• The 1969 Nobel Prize in medicine or physiology was shared by Alfred Hershey, Salvador Luria, and Max Delbrück for their discoveries concerning the mechanism of replication and the genetic structure of viruses
• Nucleotides


  The building blocks of DNA
• Nucleoside


  Contains only a nitrogenous base and a pentose sugar
• Nucleotide


  A nucleoside with a phosphate group
• Pentose sugar


  • In Ribonucleic acid (RNA)
  • In Deoxyribonucleic Acid (DNA)
• Nitrogenous Bases


  The nitrogenous base is always bonded to the 1' carbon on the pentose sugar
• Phosphate group


  Nucleotides can have one, two, or three phosphate groups attached and are called NMPs, NDPs, and NTPs, respectively
• Polynucleotide Strands


  Nucleotides are connected by a phosphodiester bond (or linkages) in which 5'-phosphate group of one nucleotide is bonded to the 3'-hydroxyl group of the next
• All polynucleotide strands have polarity, created by the free phosphate group on the 5' end and a free hydroxyl group at the 3' end
• Chargaff's Rules


  • Amount of A is proportional to T
  • Amount of C is proportional to G
  • Percentage of C + G does not equal percentage of A + T
• Rosalind Franklin, using X-ray diffraction, produced images that provided information about the structure of DNA
• 1. RAY diffraction: showed a 3.4 angstrom periodicity, characteristic of a helical structure
• Francis Crick, James Watson, Maurice Wilkins receive the Nobel Prize in Physiology or Medicine in 1962 for their determination of the structure of DNA
• Watson and Crick Model

  • Two polynucleotide strands wrapped around a central axis form a right-handed (clockwise rotation) double helix
  • Major groove alternates with minor groove -> consequences for gene expression
  • Each rotation is 34 A (3.4 nm) = 10 bases
  • Strands are antiparallel with one another
• Hydrogen bonding


  Specific between the base pairs -> base pairing is specific
• Purines bind to pyrimidines

  1. T and G-C base pairing provides complementarity of two strands and chemical stability to the helix
• The virus phi X 174 infects Escherichia coli. Its base composition is as follows: A = 24.0 %, G = 23.3 %, C = 23.3 %
• Watson-Crick model of DNA structure

  • It is composed of two strands
  • The two strands are held together by H bonds between nitrogenous bases
  • The strands run parallel
  • The resultant helix is right-handed
  • The helix has a constant diameter of 20 Å
• Watson and Crick Model

  • Hydrogen bonding is specific between the base pairs -> base pairing is specific
  • Bases are flat, lie perpendicular to the axis, and 'stack', interactions between stacked bases help stabilize the molecule
• Purines bind to pyrimidines

  • A-T and G-C base pairing provides complementarity of two strands and chemical stability to the helix
  • A-T: Double bond
  • G-C: Triple bond
• This pairing explains the Chargaff's rule!
• Other DNA Structures


  • The Watson-Crick DNA model is of B-DNA, which is believed to be the biologically significant form -> forms under conditions thought to be most similar to cellular conditions
  • Under different conditions of isolation, alternative conformations of DNA are seen
  • A-DNA is slightly more compact than B-DNA
  • C-DNA, D-DNA, and E-DNA are also right-handed forms of DNA that are less compact than B-DNA
  • Z-DNA is a left-handed form that may have a regulatory role
• DNA is the genetic material for all living organisms
• DNA's structure and properties are due to the chemistry of its constituents
• Constituents of DNA


  • Paired nitrogenous bases (total of 4) that are bonded with relatively weak hydrogen bonds
  • A sugar-phosphate backbone that is bonded by strong phosphodiester bonds and has polarity with a 3' and 5' end
• Together these make-up a double-stranded, right-handed double helix with complementary antiparallel strands
• Significance of the double helix

  • Each strand carries all the information for the double helix
  • Suggests a possible copying mechanism for the genetic material
• In RNA, the sugar ribose replaces deoxyribose of DNA and uracil replaces thymine of DNA
• RNA Structure


  • RNA structure similar to DNA but single stranded
  • Most RNA form double-stranded regions as they fold into different secondary structures